The rapid progress of computors and information industries requires various methods for printing out a huge amount of recorded information.
As one recording material in this field, a silver halide photographic material having reversal processing aptitude is used. In the step of the reversal development process, the silver halide photographic material imagewise exposed is subjected to a first development to form negative images and, without being fixed, is bleached to remove reduced silver in the images. Then, the photographic material containing undeveloped remaining silver halide is exposed to light and subjected to a second development to form positive images. In the process, the finishing speed of film is slow since the processing step is complicated and also the maximum density (Dmax) and the minimum density (Dmin) are variable. Furthermore, in the process, it is necessary to use a strong oxidizing agent such as potassium bichromate, for bleaching, which causes a problem of environmental pollution.
As a process capable of solving such a problem, a photographic process of obtaining direct positive images without need of the reversal processing step or a negative film is well known.
Processes of forming positive images using a direct positive silver halide photographic material can be mainly classified into the following two types from the viewpoint of practical usefulness.
In one type, previously fogged silver halide emulsions are used and after development, direct positive images are obtained by destroying fogged nuclei (latent images) at exposed portions by utilizing a solarization or Herschel effect.
In another type, unfogged internal latent image type silver halide emulsions are used and direct positive images are obtained by applying surface development after or while applying fogging treatment after image exposure.
The internal latent image type silver halide photographic emulsion is a silver halide photographic emulsion of a type having the sensitive specks mainly in the inside of the silver halide grain and forming latent images mainly in the inside of the silver halide grain by light exposure.
The latter process has generally high sensitivity as compared with the former process and is suitable for uses requiring high sensitivity. The present invention relates to the latter process.
Various techniques are known in the field of the art and are mainly described, e.g., in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276, 3,708,298, 3,37,322, 3,206,313 and 3,796,577, British Patents 1,150,553, 1,151,363, and 1,011,062.
By using these known processes, direct positive type photographic light-sensitive materials having relatively high sensitivity can be prepared.
Details of the mechanism of direct positive image formation are described in T. H. James, The Theory of the Photographic Process, 4th Edition, Chapter 7, pages 182-193 and U.S. Pat. No. 3,761,276.
It is believed that fogged nuclei are selectively formed at the surfaces only of silver halide grains at the unexposed portions by surface desensitizing action based on the internal latent images formed in the inside of the silver halide grains by an initial imagewise exposure, and then an ordinary surface development process is applied to form photographic images (direct positive images) at the unexposed portions.
As described above, as a means for selectively forming fogged nuclei, a "light fogging method" of applying a second light exposure on the whole surface of the light-sensitive material (e.g., British Patent 1,151,363) and a "chemical fogging method" using a nucleating agent are known. The latter method is described, e.g., in Research Disclosure, Vol. 151, No. 15162 (November, 1976), pages 72-87.
The conventional chemical fogging method has the following faults. In the method, the nucleating agent used shows the effect as a nucleating agent at a high pH of at least 12 and hence under such high pH conditions, the developing agent is liable to be deteriorated by air oxidation, which results in greatly reducing the development activity. Also, a long time is required for processing due to the slow development rate, and if a developer of low pH is used, the processing time is further prolonged. Furthermore, even when the pH of the developer is higher than 12, it takes a long time for finishing the development.
On the other hand, the light fogging method does not require a high pH condition and is relatively advantageous for practical use. However, for various purposes in a wide field of photography, there are various technical problems in this method. Since the light fogging method is based on the formation of fogged nuclei by the photodecomposition of silver halide, the proper exposure illuminance and the proper exposure amount in the method differ according to the kind and the characteristics of the silver halide being used. Thus, it is difficult to obtain constant performance in the method. Furthermore, the development apparatus is complicated and expensive. Still further, a long time is required for the development.
As described above, it is difficult to stably obtain good direct positive images by conventional fogging methods. For solving these problems, compounds showing nucleating action even below pH 12 are proposed in JP-A-52-69613 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and U.S. Pat. Nos. 3,615,615 and 3,850,638. However, these nucleating agents have the fault that the nucleating agents act with silver halide or decompose during the storage of the photographic light-sensitive materials containing them before processing, which results in reducing the maximum image density after processing.
U.S. Pat. No. 3,227,552 discloses that the development rate for an intermediate density is increased by using a hydroquinone derivative. However, even in the case of the hydroquinone derivative, the development rate is not sufficiently high and in particular, when the pH of the developer is below 12, the development rate is insufficient.
Also, JP-A-60-170843 discloses that the maximum image density is increased by adding a mercapto compound having a carboxylic acid group or a sulfonic acid group. However, the effect obtained by the addition of the compound is limited.
JP-A-55-134848 discloses that the minimum image density is reduced by processing with a processing solution (pH 12.0) containing a tetraazaindene series compound in the presence of a nucleating agent to prevent the formation of re-reversed negative images. However, in the method, the maximum image density is not increased and also the development rate is not increased.
Also, JP-B-45-12709 (the term "JP-B" as used herein means an "examined published Japanese patent application") corresponding to U.S. Pat. No. 3,708,298 discloses that a triazoline-thione series compound or a tetrazoline-thione series compound is added to a light-sensitive material for forming direct positive images by a light fogging method. However, even by this method, it is difficult to obtain high maximum image density and a high development rate.
As described above, a technique of obtaining direct positive images having high maximum image density and low minimum image density in a short period of time is not known.
Also, there is generally a problem that as the sensitivity of a direct positive silver halide emulsion increases, the formation of re-reversed negative images in a high illuminance exposure increases. In particular, in a light-sensitive film for COM, a high sensitivity in a short light exposure time is required and the prevention of the formation of the re-reversed negative images under a high illuminance exposure is important.
On the other hand, as the light source for a light-sensitive film for COM, a cathode ray tube (CRT) has been used but for improving image quality, light-sensitive materials using a laser light source (in particular, He-Ne laser source of 633 nm) have been developed.
It is well known that laser light gives a high image quality owing to its coherency, and a direct positive silver halide photographic material having a high sensitivity for use with laser light has been strongly desired.
In the field of the art, a He-Ne gas laser has been widely used from the point of reliability, but is insufficient since the device for the laser is large, its life is short, and its cost is high. On the other hand, a semiconductor laser is small and low in cost, and the laser light can be easily modulated and its life is longer than the He-Ne gas laser. Also, since the semiconductor laser emits infrared light, a bright safelight can be used for light-sensitive materials and the use of the semiconductor laser has advantage that the handling and working properties of light-sensitive materials are improved.
Thus, it has been strongly desired to develop a direct positive silver halide photographic material having a high sensitivity in the infrared region and having an excellent storage stability.